The process of implementing a damage detection and characterization strategy for engineering structures is referred to as Structural Health Monitoring (SHM). Here damage is defined as changes to the material and/or geometric properties of a structural system, including changes to the boundary conditions and system connectivity, which adversely affect the system's performance. The SHM process involves the observation of a system over time using periodically sampled dynamic response measurements from an array of sensors, the extraction of damage-sensitive features from these measurements, and the statistical analysis of these features to determine the current state of system health.
Selective interrogation is considered as a critical enabling technology for the implementation of the next generation of ultrasonic based Structural Health Monitoring systems. The ability to send ultrasonic energy in a preferential direction leads to increased damage sensitivity due to improved interaction (either in terms of back-scattered echo for a linear damage or of the nonlinear harmonic amplitude for nonlinear incipient damage) between the interrogation signal and the damage. In highly directional or anisotropic material, such as for layered composite structures, the direction of energy propagation can be largely different from the original direction of the interrogation signal. This situation results in reduced damage sensitivity because only a fraction of the incident wave energy can effectively reach the damage. The ability to generate highly directional and collimated signals can be exploited to compensate for this intrinsic characteristic of the material. In case of a multiple damage scenario, a directional interrogation would also allow to selectively scan the structural element and acquire data from the individual damage, which will increase the sensitivity and provide additional information for damage localization.
To-date, one of the most diffused approach to achieve selective interrogation for SHM applications has certainly been based on Phased-Arrays (PA) technology. PA exploits a set of transducers activated according to pre-defined time delays in order to produce either directional wavefronts or focused excitation at a prescribed spatial location. Although a robust and, to some extent, effective technology PA exhibits two important limitations that prevent its extensive use in practical applications. The first limitation consists in the large number of transducers required for implementation. The need for an extended transducer network is regarded as a major limitation in SHM applications because strictly related to increased probability of false alarms and hardware malfunctions as well as higher system complexity that affects fabrication and installation (e.g. harnessing, powering, etc.). The second major drawback of PA technology is related to its inability to generate collimated signals. In PAs, either the directional or focused excitation is the result of constructive interference produced by the superposition of multiple omni-directional wavefronts. In a multiple damage scenario, these wavefronts produce multiple reflected echoes (although weaker than those generated at the focal point) that reduce the accuracy of the detection. Note that, when multiple damages are closely spaced together (i.e. clustered damage) the damage signature does not provide the level of spatial resolution necessary to discern the individual damage. This situation typically results in an overestimated damage footprint and in lack of information about the damage shape.